1. Field of the Invention
The present invention relates to a modem apparatus using an xDSL technology that allows high-speed communications of several M bits/sec even with a telephone copper wire cable, and more particularly, to a modem apparatus, communication apparatus and communication control method that detect a CP (Cyclic Prefix) signal added to every data unit (a predetermined number of samples) of initializing signals.
2. Description of the Related Art
Against a background of the widespread proliferation of the Internet, there is a growing demand for high-speed access channels available for constant connection of the Internet. Furthermore, optical fibers are increasingly being introduced for backbone of carriers and use of ultra high-speed channels of giga-bit class is beginning in their core sections. On the other hand, most of subscriber channels connecting user residences and a carrier station are copper wire cables installed for telephones. Therefore, the introduction of an xDSL technology, which allows high-speed communications of several M bits/sec with telephone copper wire cables, is under study.
One of the xDSL technologies is an ADSL system. The ADSL system uses carrier frequencies in a band of 35 kHz or higher, which is by far higher than the band used for telephones (4 kHz or below). For this reason, the ADSL system has an advantage of using telephone lines to carry out high-speed data communications without impairment of the telephone functions.
A voice modem using a band of 4 kHz or below sends a training signal prior to data transmission and then sends a data signal. An ADSL modem sends an initializing signal, which is equivalent to the training signal, and then sends a data signal.
FIG. 7 shows a sequence diagram of an initializing signal sent by the ADSL modem. As shown in FIG. 7, the initializing signal has a CP (Cyclic Prefix) signal added at the beginning of every data unit (256 samples in the case of G.Lite) starting at some midpoint of the signal. The CP signal is configured by the same data as that of a predetermined number of samples (16 samples in the case of G.Lite. G.Lite is one technology of the ADSL system and is standardized as G.992.2 by ITU-TS in 1999.) of the rear end of the data unit. That is, the 16 samples of the rear end of the data unit are copied and added at the beginning of the data unit, forming a unit of 272 samples (256+16) as a whole. Adding this CP (Cyclic Prefix) signal at the beginning of every data unit also when a data signal is sent will prevent inter-code interference between data units. This also makes it possible to precisely demodulate a DMT (Discrete Multi Tone) modulated signal adopted by the ADSL system.
However, regardless of high-speed communication based on the ADSL system, the reception terminal cannot determine from which part of the initializing signal a cyclic insertion of the CP (Cyclic Prefix) signal starts. For this reason, it is difficult to directly recognize the boundary between the CP (Cyclic Prefix) signal and signal body. Here, the cyclic insertion refers to adding the rear 16 samples of the data unit at the beginning of the relevant data unit and repeating this procedure for every data unit.